1. Field of the Invention
The present invention relates to an apparatus for computing the relative velocity between sprung and unsprung structures constituting factors of damping-force control in a suspension system using a variable damping-force shock absorber.
2. Description of the Prior Art
The suspension of a vehicle is a system for connecting a sprung structure such as a chassis with an unsprung structure such as a wheel, which supports the sprung structure and greatly influences vibration, riding comfort, and maneuvering stability. The suspension includes such parts as a spring for moderating shocks from a road and a shock absorber for damping free vibration of the spring and controlling the velocity of attitude transition of the vehicle. As described above, there are various requirements for the suspension. However, these requirements conflict with each other. Therefore, in the case of design, an attempt is made to find a compromise by considering these requirements.
In order to achieve the above suspension requirements at a high level, a technique is known which makes it possible to vary suspension characteristics. For example, there is a technique for varying the damping forces of a shock absorber in accordance with the state of a vehicle. For example, in a frequency region near a sprung-structure resonance frequency, damping force is increased so as to converge the free vibration in its early stage, otherwise in regions other than the above region, the damping force is decreased so as to absorb an input from the road surface by the suspension much more.
This type of variable damping-force suspension is disclosed in the official gazette of Japanese Patent Laid-Open Publication No. Hei 6-106937. According to the art disclosed in this official gazette, a varying value of the damping force of a shock absorber is obtained to control the damping force by assuming that the damping force is generated proportionally to a certain parameter indicating a vehicle moving state shown by the vertical velocity of a sprung structure. By controlling the damping force, it is possible to decrease the number of sensors needed to detect the vehicle moving state. Moreover, to obtain the parameter showing the vehicle moving state, operations are performed by assuming that each operation factor varies linearly.
As the characteristics of the damping force of a shock absorber, the damping force is generated depending on the velocity of a stroke including the direction and the valve opening degree of an orifice portion. According to the art disclosed in the official gazette, the damping force is generated proportionally to the parameter. Actually, however, the damping force is not generated proportionally to the parameter. Therefore, there is a problem that the damping force cannot necessarily be controlled at a high accuracy. Moreover, there is a problem that the parameter cannot be computed by a linear observer at a high accuracy because each operation factor actually changes in a nonlinear manner.